Abstract: We present late-time optical $R$-band imaging data from the Palomar Transient
Factory (PTF) for the nearby type Ia supernova SN 2011fe. The stacked PTF light
curve provides densely sampled coverage down to $R\simeq22$ mag over 200 to 620
days past explosion. Combining with literature data, we estimate the
pseudo-bolometric light curve for this event from 200 to 1600 days after
explosion, and constrain the likely near-infrared contribution. This light
curve shows a smooth decline consistent with radioactive decay, except over
~450 to ~600 days where the light curve appears to decrease faster than
expected based on the radioactive isotopes presumed to be present, before
flattening at around 600 days. We model the 200-1600d pseudo-bolometric light
curve with the luminosity generated by the radioactive decay chains of
$^{56}$Ni, $^{57}$Ni and $^{55}$Co, and find it is not consistent with models
that have full positron trapping and no infrared catastrophe (IRC); some
additional energy escape other than optical/near-IR photons is required.
However, the light curve is consistent with models that allow for positron
escape (reaching 75% by day 500) and/or an IRC (with 85% of the flux emerging
in non-optical wavelengths by day 600). The presence of the $^{57}$Ni decay
chain is robustly detected, but the $^{55}$Co decay chain is not formally
required, with an upper mass limit estimated at 0.014 M$_{\odot}$. The
measurement of the $^{57}$Ni/$^{56}$Ni mass ratio is subject to significant
systematic uncertainties, but all of our fits require a high ratio >0.031 (>1.3
in solar abundances).